Dental implants which are inserted into the jawbone, for example for the attachment of artificial teeth, have been successfully used for more than a decade. The major part of the dental implants currently used consist of titanium, since titanium is biocompatible, has a sufficiently low elastic modulus and a relatively high strength.
Apart from its biocompatibility and its mechanical properties, the osteointegrative properties of a dental implant are of major importance. A good osteointegration means that the implant, after reaching a primary stability by screwing it into the bone, safely ossifies within a short healing time so that a permanent bond between implant and bone is guaranteed.
When using titanium as the implant material, osteointegrative properties can be reached by suitable treatment of the implant's surface. To this end, the titanium surface has conventionally been mechanically roughened by a subtractive removing process, e.g. sandblasting, grinding or etching. Alternatively, the surface has been subjected to additive processes, e.g. coating with a textured surface.
U.S. Pat. No. 6,174,167 discloses implants having a surface for bone-tissue apposition, said surface being obtained by machining, application of a textured surface or blasting with particles. It also discloses acid etching, applying growth factor, protein or other materials that promote, enhance and/or maintain bone-tissue growth and/or apposition. The implant is made from a biocompatible material, preferably from titanium or an alloy thereof.
DE-A-4012731 describes a process for producing an implant made of titanium employing spark erosive techniques in order to provide a desired roughness to the implant's surface.
Osteointegration has turned out to be particularly efficient if mechanical roughening of the implant's surface is combined with subsequent etching of the roughened surface, as is for example described in Li et al., J. Biomed. Mater. Res. 2002, 60 (2), pages 325-332.
Similarly, EP-A-0388576 describes the treatment of a titanium implant by a blasting process and subsequent etching with a reducing acid, such as HF, HCl or HCl with H2SO4.
Conventional titanium implants which are subjected to such a combined treatment safely ossify within a healing time of about 3 to 4 months after insertion into the bone, thereby providing a permanent bond between the dental implant and the bone.
From an aesthetic point of view, titanium implants have, however, the disadvantage that they are dark in color and therefore mismatch with the natural tooth color.
In contrast, the color of ceramic materials can be closely matched to the natural tooth color. Efforts have thus been made to provide dental implants of which at least the parts that are visible after insertion are made of a ceramic material.
WO-A-0134056 refers to a dental implant consisting of an insertion component which can be fitted in the jawbone and a support component which after implantation protrudes beyond the jawbone. The document discloses that at least the support component is made of zirconia ceramic.
DE-A-19530981 refers to a prefabricated implant supra construction using a tooth-colored zirconia ceramic attached to titanium implants.
Despite their advantageous properties with regard to the color, the use of ceramic materials for dental implants have been limited by their low fatigue stability and thereby by their tendency to crack.
A ceramic material having a high mechanical strength is disclosed in U.S. Pat. No. 6,165,925. U.S. Pat. No. 6,165,925 relates to an yttrium-stabilized zirconium oxide (zirconia) in predominantly tetragonal form for the production of a sintered semi-finished article as a starting material for the manufacture of a prosthesis.
In order to achieve a sufficient mechanical stability, the zirconia ceramic disclosed in U.S. Pat. No. 6,165,925 must be highly dense. The surface of said highly dense zirconia ceramic is clean cut, extremely hard and has essentially no porosity. A dental implant made of such a zirconia ceramic is thus bio-inert and has only weak osteointegrative properties. Conventional techniques for treating the implant's surface in order to achieve a osteointegrative surface have failed, mostly due to the extreme hardness of the material.
Other techniques for providing an osteointegrative ceramic surface have been suggested:
WO-A-2005/027771 relates to a process for preparing a dental installation in which a dispersion is applied on a substrate having a first porosity, said dispersion forming upon sintering a ceramic layer with a second porosity.
EP-A-0870478 relates to a dental retention element having a core of a high-strength material such as zirconia, said core being coated with a ceramic material which can be chemically and/or mechanically processed.
The composite structures disclosed in WO-A-2005/027771 and EP-A-0870478 have the disadvantage that the ceramic coating is easily chipped off.
Alternatively, a treatment comprising abrasive blasting and acid-etching for providing osteointegrative properties to the ceramic implant's surface has been suggested by EP-B-1450722. EP-B-1450722 relates to a dental implant made of zirconia ceramic which after abrasive blasting is subjected to a treatment using phosphoric acid, sulphuric acid, hydrochloric acid or mixtures thereof.
However, the level of osteointegration of such zirconia implants revealed by their removal torque values have been shown to be lower compared to the conventionally used titanium implants. Although after implantation, an initial increase of the removal torque values of the zirconia implants could be measured, a decrease was observed after a certain period. Corresponding studies are discussed in Gudehus, H. T.; Untersuchung des Einwachsverhaltens von Zirkoniumdioxid-Implantaten in die Kieferknochenstruktur—Eine experimentelle Studie am Miniaturschwein, Dissertation Ludwig-Maximilians-Universität München, 2006.
Lately, Takemoto M. et al. (J. Biomed. Mater. Res., 2006, 78A, pages 693-701) have published a report on the bone-bonding ability of a hydroxyapatite coated zirconia-alumina composite which prior to the coating with the hydroxyapatite is treated at 60° C. in hydrofluoric acid, then washed and heat-treated at 1300° C. The process requires a specific and sophisticated apparatus for the heat-treatment of the implant.